1. Field of the Invention
The present invention relates to a method of forming a p-n junction on a ZnO thin film and a p-n junction thin film, and more particularly, to a method of forming a p-n junction on a ZnO thin film and a p-n junction thin film which can fabricate a LED device by depositing Zn3P2 on the ZnO thin film and forming it into an effective p-type material.
2. Description of the Related Art
In our technical world displays have an important function as human interfaces for making abstract information available through visualization. In the past, many applications for displays were identified and realized, each with its own specific requirements. Therefore, different display technologies have been developed, each having their own strengths and weaknesses with respect to the requirements of particular display applications, thus making a particular display technology best suited for a particular class of applications.
Light emitting diodes (LED) which emit light spontaneously under forward bias conditions have various fields of application such as indicator lamps, devices of visual displays, light sources for an optical data link, optical fiber communication, etc.
In the majority of applications, either direct electronic band-to-band transitions or impurity-induced indirect band-to-band transitions in the material forming the active region of the LED are used for light generation. In these cases, the energy gap of the material chosen for the active region of the LED, i.e. the zone where the electronic transitions responsible for the generation of light within the LED take place, determines the color of a particular LED.
A further known concept for tailoring the energy of the dominant optical transition of a particular material and thus the wavelength of the generated light is the incorporation of impurities leading to the introduction of deep traps within the energy gap. In this case, the dominant optical transition may take place between a band-state of the host material and the energy level of the deep trap. Therefore, the proper choice of an impurity may lead to optical radiation with photon energies below the energy gap of the host semiconductor.
Today, exploiting these two concepts for tailoring the emission wavelength of an LED and using III-V or II-VI compound semiconductors or their alloys for the active region of the LED, it is possible to cover the optical spectrum between near infrared and blue with discrete emission lines.
Blue light emitting MIS diodes have been realized in the GaN system. Examples of these have been published in:
xe2x80x9cViolet luminescence of Mg-doped GaNxe2x80x9d by H. P. Maruska et al., Applied Physics Letters, Vol. 22, No. 6, pp. 303-305, 1973,
xe2x80x9cBlue-Green Numeric Display Using Electroluminescent GaNxe2x80x9d by J. I. Pankove, RCA Review, Vol. 34, pp. 336-343, 1973,
xe2x80x9cElectric characteristics of GaN: Zn MIS-type light emitting diodexe2x80x9d by M. R. H. Khan et al., Physica B 185, pp. 480-484, 1993,
xe2x80x9cGaN electroluminescent devices: preparation and studiesxe2x80x9d by G. Jacob et al., Journal of Luminescence, Vol. 17, pp. 263-282, 1978,
EP-0-579 897 A1: xe2x80x9cLight-emitting device of gallium nitride compound semiconductorxe2x80x9d.
Unfortunately, the present-day LEDs suffer from numerous deficiencies. Light emission in the LED is spontaneous, and, thus, is limited in time on the order of 1 to 10 nanoseconds. Therefore, the modulation speed of the LED is also limited by the spontaneous lifetime of the LED.
Attempts were made to improve the performance of the LEDs. For example, a short wavelength blue semiconductor light emitting device has been developed. The compound semiconductor device of gallium nitrite series such as GaN, InGaN, GaAlN, InGaAlN has been recently considered as a material of the short wavelength semiconductor light emitting device.
For example, in the semiconductor light emitting device using GaN series material, a room temperature pulse oscillation having wavelength of 380 to 417 nm is confirmed.
However, in the semiconductor laser using GaN series material, a satisfying characteristic cannot be obtained, a threshold voltage for a room temperature pulse oscillation ranges from 10 to 40V, and the variation of the value is large.
This variation is caused by difficulty in a crystal growth of the compound semiconductor layer of gallium nitride series, and large device resistance. More specifically, there cannot be formed the compound semiconductor layer of p-type gallium nitride series having a smooth surface and high carrier concentration. Moreover, since contact resistance of a p-side electrode is high, a large voltage drop is generated, so that the semiconductor layer is deteriorated by a heat generation and a metal reaction even when the pulse oscillation is operated. In consideration of a cheating value, the room temperature continuous oscillation cannot be achieved unless the threshold voltage is reduced to less than 10V.
Moreover, when a current necessary to the laser generation is implanted, the high current flows locally and a carrier cannot be uniformly implanted to an active layer, and an instantaneous breakage of the device occurs. As a result, the continuous generation of the laser cannot be achieved.
In the light-emitting device of GaN series, since the p-side electrode contract resistance was high, the operating voltage was increased. Moreover, nickel, serving as a p-side electrode metal, and gallium forming the p-type semiconductor layer, were reacted with each other, melted, and deteriorated at an electrical conduction. As a result, it was difficult to continuously generate the laser.
Besides, SiC and ZnO are known as short wavelength light emitting materials.
However, SiC and ZnO are disadvantageous in that the chemical crystalline thereof is very unstable or a crystal growth itself is difficult for SiC and ZnO to be used as compounds semiconductors required for blue light emission. In case of SiC, it is chemically stable, but the lifetime and brightness thereof are low for SiC to be put into practical use.
Meanwhile, in case of ZnO, it is proper material for blue light emission and shorter wavelength light emission since it has a characteristic similar to GaN. Moreover, ZnO has an excitation binding energy (e.g., 60 meV) about three times larger than that of GaN, it is judged to be a very proper material for short wavelength light element of the next generation.
Nevertheless, even though there was a case where ZnO was fabricated as a p-n junction, the light emission efficiency thereof was very low and thus the availability thereof as an actual device is very low, and it is difficult for ZnO to form a p-type material.
It is, therefore, an object of the present invention to provide a method of forming a p-n junction on a ZnO thin film and a p-n junction thin film which deposits Zn3P2 on a ZnO thin film and forms a p-type material constituting a device by using thermal diffusion for the Zn3P2 in order to fabricate an effective p-type material.
To achieve the above object, there is provided a method of forming a p-n junction on a ZnO thin film on a sapphire base substrate for use in a light emitting device in accordance with a preferred embodiment of the present invention, comprising the steps of: cladding the sapphire substrate with a n-type ZnO thin film; depositing a Zn3P2 thin film on the n-type ZnO thin film; forming a p-type ZnO thin film by irradiating a laser on the upper surface of the Zn3P2 thin film, decomposing the Zn3P2 thin film and diffusing the same on the n-type ZnO thin film; and forming an electrode on the n-type ZnO thin film and the p-type ZnO thin film respectively.
Preferably, there is provided a method of forming a p-n junction on a ZnO thin film which fabricates a multi-layer light emitting device of npn-type or pnpn-type by repeatedly forming a n-type ZnO thin film and a p-type ZnO thin film on the sapphire substrate.
Meanwhile, there is provided a p-n junction thin film on a sapphire base substrate for use in a light emitting device in accordance with the present invention, comprising: a n-type ZnO thin film on the sapphire substrate formed by a cladding; a Zn3P2 thin film deposited on the n-type ZnO thin film; a p-type ZnO thin film by irradiating a laser on the upper surface of the Zn3P2 thin film and decomposing the Zn3P2 thin film and diffusing the same on the n-type ZnO thin film; and an electrode on the n-type ZnO thin film and the p-type ZnO thin film respectively.